1. Field of the Invention
The present invention relates to an annular sliding fluoroplastics member which is required to have good mechanical properties, resistance to abrasion and wear, thermal conductivity, hear resistance, and the like, and more particularly to an annular sliding fluoroplastics member which can be preferably used as a radial slide bearing, a thrust washer, or the like.
The present invention relates also to a method of producing an annular sliding fluoroplastics member which can produce such an annular sliding fluoroplastics member by means of simple steps.
2. Description of the Prior Art
As an annular sliding fluoroplastics member which is used as a radial slide bearing, a thrust slide bearing, a thrust washer, or the like, known are three types of annular sliding fluoroplastics members which will be described below.
An annular sliding fluoroplastics member of a first prior art example is molded by singly pressurizing and firing powder of granular fluoroplastics such as PTFE (Polytetrafluoroethylene) plastics.
An annular sliding fluoroplastics member of a second prior art example is molded by pressurizing and firing a complex which is obtained by dry mixing short fibers such as chopped aramid fibers or powder of plastics with PTFE plastics.
An annular sliding fluoroplastics member of a third prior art example is formed in the following manner. Short fibers made of fibrillated aramid plastics or the like, and PTFE plastics are uniformly wet mixed by, for example, a mixer. The wet mixed mixture is formed into sheet-like elements. A plurality of such sheet-like elements are stacked to form a layered structure. The layered structure is fixed and then subjected to various machining works such as a cutting work, to be formed into an annular shape.
The annular sliding fluoroplastics member of die first prior art example is excellent in resistance to abrasion and wear. In the member, however, thermal conductivity is poor in the case where the fluoroplastics and die counter member directly slide over eadi other to generate heat. Therefore, seizure easily occurs in the slide face, and hence it is difficult to maintain the sliding property stable for a long term.
In the annular sliding fluoroplastics member of the second prior art example, it often happens that the short fibers are not uniformly mixed. In such a case, high-density portions of short fibers and low-density portions of short fibers exist in a mixture in the slide face. In a low-density portion of short fibers, the sliding area between the fluoroplastics and the counter member is increased so that, in the same manner as the annular sliding fluoroplastics member of the first prior art example, the thermal conductivity of the low-density portion of the short fibers is lowered. As a result, seizure easily occurs in the slide face, and hence it is difficult to stably maintain the sliding property for a long term. In a thrust slide bearing and a thrust washer, since a large press load is applied in the axial direction, it is preferable to orient the short fibers in the axial direction along which the magnitude of the load is large, thereby enhancing the buckling resistance. By contrast, in a radial slide bearing, since a large press load is applied in a radial direction, it is preferable to orient the short fibers in a direction which is as close as possible to the radial direction so as to enhance the pressure resistance in a radial direction. In the annular sliding fluoroplastics member of the second prior art example, however, the short fibers are randomly oriented, and hence the buckling resistance, and the pressure resistance in a radial direction are so low that the annular sliding member has a low mechanical strength.
The annular sliding fluoroplastics member of the third prior art example is produced by stacking plural sheet-like elements and cutting the resulting layered structure into an annular shape. Therefore, the production steps are complicated, and a large amount of chips must be disposed. As a result, the materials are wastefully used and the production cost is increased. Furthermore, most of the short fibers in the sheet-like elements are oriented substantially in one direction, and hence the orientation of the short fibers is restricted to a radial direction which is parallel to the radial direction, or is not always coincident with the direction along which the burden of a load is large. Therefore, it is difficult to employ the method in which the orientation of the short fibers is restricted so as to improve the buckling resistance, and the pressure resistance in a radial direction, thereby enhancing mechanical strength.